1. Field of the Invention
The present invention relates to an electric power steering system entrusting a steering power to the drive of an electric motor.
2. Description of the Prior Art
The system of this kind according to the prior art is exemplified, as shown in FIG. 4, by the system proposed by Japanese Patent Laid-Open No. 227560/1984.
This system will be summarized with regard to the use of the system on a detecting unit for detecting the turning direction and steering force of a steering shaft. A steering shaft 36 and a worm shaft 33 are coupled to the two ends of a torsion bar 37 by means of pins P.sub.1 and P.sub.2, respectively. To the leading end of the steering shaft 36, there is fixed a slider 41 contacting with a sliding resistor 40, both of which constitute a sensor 42 for detecting the turning direction and steering force of the steering shaft 36. Incidentally, reference numeral 31 denotes a steering gear box, in which are accommodated the worm shaft 33, a ball nut 32 threaded on the worm shaft 33 through steel balls 34, a sector shaft 35 having a pinion 35a meshing with a rack 32a formed on one side face of the ball nut 32, and the aforementioned sensor 42. In one end face of the steering gear box 31, on the other hand, there is formed a hole, through which extends one end of the worm shaft 33 connected to an electric motor 39 through a reduction gear mechanism 38.
Now, if the not-shown steering wheel is actuated to turn the steering shaft 36, the torsion bar 37 is twisted to displace the slider 41 from its initial position with respect to the sliding resistor 40. The sensor 42 outputs a change in the resistance as an output S.sub.1 to a controller 43. In response to this signal S.sub.1 and a signal S.sub.2 from a vehicular speed sensor 44, the controller 43 outputs a drive signal S.sub.3 to the electric motor 39. Then, this electric motor 39 revolves while having its turning direction and force controlled by the drive signal S.sub.3 to generate a steer assisting force.
Incidentally, in the power steering system of the type in which the steering force and the turning direction are detected in terms of the torsion of the torsion bar, the so-called "centering operation" is required for strictly adjusting the neutral position so as to warrant the transverse symmetry of the characteristics. Especially in the electric power steering system, it is important to adjust the output signal level of the sensor in the neutral position to the reference level of the input signal of the controller.
In this electric power steering system of the prior art including the aforementioned system, the sensor is made integral with the constructional components such as the steering shaft or the gear so that the aforementioned centering operation is large-scaled and it is required to have highly precise machining, thus causing a rise in the production cost.
In order to solve these problems, it is conceivable to rough the centering operation of the sensor so that the reference level of the input signal at the side of the controller may be adjusted to the output signal level in the neutral state of the sensor. In this case, the interchangeability of the controller is lost thus making it necessary to handle the sensor and the controller always in a pair. This raises a demerit of losing the interchangeability from the controller. Therefore, this concept cannot be adopted at all.
Thus, the electric power steering system of the existing type, in which the turning force and direction of the input/output shafts are detected to control the revolutions of the electric motor by making use of the relative revolutions of them, is accompanied by the problem that, since the components of the steering gear and the sensor are integrated, the centering operation of the sensor unit is large-scaled and required to have the highly precise machining.